Monday, April 27, 2015

Why do plants flare, anyway?

In the article “Environmental Protection Agency, Refiners Clash Over Hazardous Air Pollution,” I took particular note of the following sentence: EPA “would also strengthen operational requirements for flaring, the process of burning off and destroying excess hydrocarbon gases” (C&EN, Jan. 26, page 27).

Why does anyone flare anything anymore? These hydrocarbon gases could be used as raw materials in refinery processes. At the very least, they could be burned to produce energy for processes within the refinery instead of wasting the energy by releasing it into the air. What am I missing?

Allen HoffmanNew York City

I would also really like to know the answer to this question. (I presume the cost of capturing the gas is more expensive than the cost of flaring, but I dunno.)

9 comments:

Two possibilities:1. Methane is extremely difficult to store and transport owing to its low boiling point. Sometimes it is converted to syngas but that is a very energy and capital intensive process. Oil rigs may be located in very remote areas where methane capture and transport/conversion is uneconomical.2. Flaring is a safety feature. In case of a dangerous buildup of gases somewhere in the plant, to prevent explosions flaring allows for such gases to be safely released with a minimum of environmental damage.

Flaring is crazy if you happen to be near a gas pipeline that leads to a refinery. Unfortunately, most natural gas discovered as part of the shale boom did not meet this criterion. The Department of Energy and others have funded a variety of projects to make capturing or storing natural gas cheaper. Approaches have included (i) chemical or biochemical conversion to methanol, to syngas, to ethylene, or to biomass via methanotrophic bacteria; (ii) reducing costs of compressed natural gas compression and storage. Option (ii) has met with some commercial success. An NYT article from the boom times of 2013 (http://www.nytimes.com/2013/12/18/business/energy-environment/applying-creativity-to-a-byproduct-of-oil-drilling-in-north-dakota.html) talks a bit about the problem and the inroads that a General Electric product made:

The first step of the process is to strip out of the gas valuable natural gas liquids like butane and propane, which can be used for petrochemical production. The liquids can then be put in pressurized tanks and delivered by truck to processing plants. The rest of the gas can be compressed and stored in what G.E. calls “C.N.G. in a box.”

...

By Statoil’s calculations, if all the rigs in the Bakken were converted to run even partly on natural gas, more than 60 million cubic feet of natural gas — or roughly a fifth of the gas now being flared — could be saved every day.

...

The costs of the program are modest. A General Electric compression box costs about $1.1 million, and the mobile processor costs about $500,000, according to Statoil executives. The company hopes to get the first pilot running by early January and have up to eight compression units running by the end of 2014.

Keep in mind that this was the plan back when oil was way more expensive than now. Natural gas is still cheap, but it's unclear to me if the economic factors that would favor gas capture over flaring are still the same now that oil is cheap. (The money to pay back the purchase capital for the capture equipment would presumably come from some uncertain mix of oil profits (lower now) and gas profits (zero if you don't capture, nonzero if you do). I think oil companies have made commitments to the EPA reduce the total amount or fraction of gas that they flare.

The chemical conversion approaches are interesting; many of the thermochemical approaches would require huge refineries to be profitable. It just isn't feasible (yet) to build the high-pressure, high-temperature reactors that do gasification in a small, modular way such that they can be distributed to many different remote wellhead locations at low cost. The biochemical approaches don't have that problem -- they work at ambient temperatures and pressures -- but they tend to need a lot of water.

if you want to make LNG or syngas, you need really an elaborate, investment-demanding plant that operates at high pressures and fairly high temperature or pretty low temperatures, hence it favors economy of scale.

One argument for research into methane catalytic oxidation with air to methanol is that the process could possibly run in relatively small units - so this kind of marginal sources of natural gas that are flared could be turned into something useful

Chemical plants and refineries flare during process upsets. It's infrequent and, most importantly, inconsistent flow of fuel so it cannot be used to feed the generators.

Oil rigs flare gas that has not have been processed so it's "dirty" and may damage the generator. Environmental policies are possibly at play too - burning dirty gas in a flare is OK but running the same gas through electricity generator is not OK because emissions are counted differently.

The plant I worked at only ran a flare to knock out fugitive emissions (VOCs) to meet EPA regs. Trying to capture and usefully burn a couple hundred ppm of anything, no matter how combustible it is, is not an economically sound idea.

Methane to methanol, and the whole idea of a methanol economy, are exciting frontiers in chemistry. It's a remarkably practical idea given that much of the heavy industry surrounding methanol's use as a chemical feedstock already exists. I'm excited by the progress made in oxidative coupling of methane to ethylene by Siluria. This is timely technology given that global demand for polyethylene continues to rise. Non-oxidative coupling of methane to ethane (and higher hydrocarbons) by alkane metathesis is another intriguing possibility, but technology to accomplish this is not ready for prime time. My prediction is that an array of methane conversion technologies will eventually take hold and operate in parallel to generate commodity chemicals as efficiently as possible. At least, that's my blue sky hope.

Velocys (formerly oxford catalysts) has been developing compact fischer-tropsch units designed for on-rig conversion of methane that would normally be flared into liquid HCs. It uses highly active catalysts in flow reactors (for heat dissipation). Not sure how far it's got, but we talked to them in this Chem World feature (Article is paywalled I'm afraid, but the pdf is free here http://www.rsc.org/images/Synthetic%20Fuels%20-%20Liquid%20Assets_tcm18-201247.pdf)